COLUMNS 173 



mation diagram where the rate of deformation increases rapidly. 

 It is very nearly the deformation at which plain concrete would 

 fail. Theoretically it can be shown that within this limit of 

 elasticity the hooped reinforcement is much less effective than 

 longitudinal reinforcement, but that such reinforcement may 

 be quite effective in increasing the ultimate strength of the 

 column. The results of tests verify the theoretical A conclusion. 

 The ultimate strength is found increased, but the shortening of 

 the column is so great at a comparatively early period in the 

 loading, that the safe strength cannot be based directly on the 

 ultimate strength. Concrete expands laterally only a very 

 small percentage of its vertical deformation or shortening, so 

 that the hoops do not come much into play until the concrete 

 has shortened to an extent such that its elastic limit has been 

 passed. Under further loading, however, the concrete is 

 prevented by the hooping from actual failure, but continues 

 to expand laterally until final failure occurs by the breaking of 

 the wire or by its excessive stretching. It is also found that the 

 shell of concrete outside of the hooping, which is necessary for 

 fireproofing and for the protection of the steel, begins to crack- 

 and peel off at about the same load as that which causes complete 

 failure in plain concrete columns. If hooping is not continuous 

 or rigid, it will also peel off with the surface concrete so that the 

 effective strength of the column will be no greater than a similar 

 one of plain concrete. 



67. Columns with Hooped and Longitudinal Reinforcement. 

 The addition of bands or spirals to columns having longitudinal 

 reinforcement does not have much effect upon the deformation 

 of such columns up to the point of failure without hooping. In 

 fact the elastic limit and rigidity of the column appears to be 

 decreased if anything. The effect of such hooping, however, 

 raises slightly the ultimate strength and increases the capacity 

 of the column to deform at loads beyond the elastic limit, so 

 that a somewhat higher working stress may be employed on 

 the concrete than for plain concrete columns. Tests show that 

 about 1 per cent of a closely spaced spiral of high carbon steel 

 is sufficient to prevent the longitudinal rods from bulging outward 

 and will provide a satisfactory amount of toughness with a 

 corresponding raising of the ultimate strength beyond the elastic 

 limit. Vertical reinforcement combined with spiral hooping is 

 shown in Fig. 81. 



